1. Field of the Invention
This invention relates to current sources and is more in particular directed to a matching current source for providing the same quantity of sink current as source current.
2. Description of the Prior Art
In many integrated circuits, particularly analog circuits, current sources are frequently used. For some cases, a matching current source which generates equivalent sink and source current is very important. However, it is recognized that the transistor current is affected by the channel length modulation effect in MOS devices and Early Effect in bipolar devices. The drain currents of MOS transistors are consequently not independent of their drain-source voltages and the collector currents of bipolar transistors are not independent of their collector-emitter voltages. Actually, the drain current (collector current) increases with increasing drain-source voltage (collector-emitter voltage).
FIG. 1 illustrates the circuit of a known current source which provides sink current and source current. In this circuit, MOS transistors T1 and T2 are serially connected, with the sources of the transistors T1 and T2 being connected to the voltage terminal V+ and V- respectively and the gate of transistor T1 being connected to its drain at node F. Node F is also coupled to the gate of transistor T3 having its source connected to the terminal V+. Input voltages at node B are applied to the gate of transistor T2 as well as to the gate of a transistor T4 that has its source connected to the voltage terminal V-. A voltage node A controls a switch S, for selectively connecting output node C to the drains of the transistors T3 and T4, at terminals D and E respectively.
Since the channel length modulation effect exists, this circuit can't provide equivalent sink and source current. Node B controls the amplitude of source current. Node C is an output node which is at a fixed voltage in the range between V+ and V-. When node A controls the switch S to close the contact between C and D and open the contact between C and E, this circuit functions to source current. Changes in the voltage applied to node B change the current I1. The drain-source voltage Vds1 of transistor T1 and drain-source voltage Vds2 of transistor T2 vary in opposite directions with changes in the voltage of node B. However, the voltage of node C is constant. Therefore, Vds1-Vds2, Vds1-Vds3 and Vds2-Vds4 vary with changes in the voltage of node B. Vds1, Vds2, Vds3 and Vds4 are the drain-source voltages of transistors T1, T2, T3 and T4 respectively. Not only transistors T1 and T2, but also transistors T3 and T4, experience the different degree of channel length modulation effect. A linear relationship consequently does not exist between currents I1, and I2, and a linear relationship does not exist between currents I3 and I1. Similarly a linear relation does not exist between currents I2 and I3. Moreover, the ratios of current I4(=I2) in the sourcing current mode and the current -I4(=I3) in the sinking current mode are different for different voltages at node B. Therefore, this circuit can's provide matched current. In a matched current condition the amplitude of source current in a sourcing current mode is the same as the amplitude of sink current in sinking current mode, independently of whether those currents are large or small.
One method commonly employed to overcome this problem is the adjustment of the current source by laser trimming. However, this method only provides equivalent source and sink current at one constant current, and the ratio of source-to-sink current varies with different amplitudes of source and sink current.